1. Field of the Invention
The present invention relates to a supply apparatus which supplies radicals, a lithography apparatus, and a method of manufacturing an article.
2. Description of the Related Art
Conventionally, a projection exposure method using ultraviolet light as an energy beam has been practiced as one of the lithography methods for manufacturing fine semiconductor devices such as semiconductor memories and logic circuits. With increases in the integration of semiconductor devices, however, demands have arisen for the establishment of a lithography technique of obtaining a resolution of 35 nm or less, which cannot be achieved by lithography methods using ultraviolet light of conventional wavelengths. The resolution in projection exposure is proportional to the wavelength of light used for exposure. For this reason, as one of the methods for efficient exposure of very fine circuit patterns described above, a lithography technique has been developed, which uses EUV (Extreme Ultraviolet) light having a wavelength of 5 to 20 nm shorter than conventional ultraviolet light as an energy beam.
In the EUV region, almost all substances have strong absorbance, and hence it is almost impossible to use transmission type optical elements using refraction as in lithography using conventional ultraviolet light as exposure light. An EUV exposure apparatus therefore uses reflection type optical elements such as mirrors for an energy beam irradiation system to control an energy beam. A multilayer film having two or more types of layers with different optical constants stacked on each other is formed on a surface of an optical element. As an optical element, for example, an element obtained by alternately stacking molybdenum and silicon layers on the surface of a glass substrate which is polished into a precise shape is known. In addition, attention has conventionally been given to a lithography technique of directly drawing a fine circuit pattern on a semiconductor substrate by using a charged particle beam as an energy beam.
EUV light is also absorbed by gas components existing in the atmosphere and is considerably attenuated. For this reason, the inside of an exposure apparatus is held in a vacuum high enough to prevent exposure light from being attenuated. In a lithography apparatus using an electron beam, an electron beam generation source and a beam path are also held in a vacuum. Small amounts of gases containing carbon compounds and water as main components remain in a vacuum atmosphere in the lithography apparatus. These residual gases are generated from components, cables, organic material components, and the like used inside the exposure apparatus. Volatile gases are also generated from the photosensitizing agent (to be referred to as a resist hereinafter) applied on a wafer.
A residual gas is repeatedly adsorbed and desorbed on and from the surface of an optical element used in the exposure apparatus. However, physical adsorption alone hardly cause adsorption or reaction on the optical element surface. When, however, the optical element is irradiated with EUV light, secondary electrons generated on the optical element surface cause a physically adsorbed gas to dissociate. A product due to the dissociation is sometimes deposited on the optical element surface or a generated reactive species sometimes modifies the optical element surface. They are called contaminations. Such contaminations mainly include the following two phenomena. One phenomenon is that components of residual gases which contain carbon are physically adsorbed on the optical element surface, are dissociated from the surface when irradiated with EUV light, and carbon or compounds containing it are deposited on the optical element surface. The other phenomenon is that water components adsorbed on the optical element surface are dissociated, when irradiated with EUV light in the same manner, to generate active oxygen and oxidize the optical element surface.
When carbon or a compound containing it is deposited in an amount exceeding a given amount on the optical element surface or the optical element surface is oxidized, the performance of the optical element deteriorates, resulting in a deterioration in the performance of the exposure apparatus. In an EUV exposure apparatus, in particular, if an optical element is a reflection type multilayer film mirror, even the deposition of merely several nm of a carbon compound per mirror or the oxidation of the uppermost layer to a thickness of merely several nm leads to a decrease in reflectance which considerably affects the throughput. Attempts have been made to prevent surface oxidation among these contaminations by providing an oxidation-resistant film. On the other hand, it is thought that the deposition of carbon or the like cannot be avoided to a certain degree, and hence attempts have been made to remove the deposited carbon or carbon compound.
Several methods of removing carbon deposited on an optical element have been proposed. For example, Japanese Patent Laid-Open No. 2005-244015 discloses a method of separately arranging a light source for cleaning and causing, at the time of stopping exposure, the light source for cleaning to apply light onto the same optical path as that used for exposure. This method also provides a gas supply system and performs optical cleaning while introducing an oxidizing gas such as oxygen or ozone, hydrogen, water, and the like. Introducing these gases will make a compound containing carbon react with the gases and facilitate removing the compound. Japanese Patent Laid-Open No. 2004-200686 has proposed a method of removing a carbon deposit on a multilayer film mirror by generating active atomic hydrogen (hydrogen radicals) and introducing it onto the surface of an optical element. Japanese Patent Laid-Open No. 2007-281321 has proposed a method of removing an oxide film or carbon deposition on an optical element surface by irradiating the optical element surface with exposure light while introducing a reducing gas.
Of these methods, the method using hydrogen radicals is expected as a promising method because a hydrogen radical has the ability to remove a carbon deposit and the ability to reduce a oxidized mirror surface. FIG. 4 shows an example of an arrangement for contamination removal using hydrogen radicals in an EUV exposure apparatus. In order to generate hydrogen radicals, a current is supplied to a filament 12 as an activation source to heat it to about 1,500° C. or higher, and hydrogen gas is introduced from an introduction port 13. At this time, an exhaust system 15 of the exposure apparatus performs exhausting operation. In this case, the hydrogen radicals generated in a chamber 11 pass through a transport pipe 14 and flow into a housing (or frame) 10 which holds (accommodates) mirrors installed in the exposure apparatus. The inflowing hydrogen radicals react with the carbon contamination deposited on the surfaces of mirrors 7, 8, and 9 to generate hydrocarbon gas, which is then exhausted from the exhaust system 15. This removes the carbon contamination from the mirrors 7, 8, and 9 and recovers the reflectance of the mirrors.
Since the filament 12 which generates hydrogen radicals is heated to 1,500° C. or higher as described above, if the filament 12 is directly installed in the exposure apparatus, the filament 12 applies radiation heat to the mirrors 7, 8, and 9 and the housing 10. This will disturb the optical system including the mirrors 7, 8, and 9 whose positions have been accurately adjusted. In order to avoid this, for example, as shown in FIG. 4, the chamber 11 accommodating the filament 12 is installed outside a vacuum chamber 1 accommodating the exposure apparatus to introduce generated radicals into the exposure apparatus through the transport pipe 14. Although not shown in the accompanying drawings, a vessel forming the chamber 11 can be temperature-controlled by water cooling or the like to avoid influences of heat conduction and the like.
As described above, since the filament 12 serves as a heat source, it is necessary to thermally detach it from the optical elements and other components as far as possible. The transport pipe 14 therefore needs to be long. It has been conventionally pointed out that hydrogen radicals lose their activity when they come into contact with other solids such as the wall of the chamber 11 surrounding the filament 12, the inner wall of the transport pipe 14, the surfaces of optical elements, and the components of the apparatus. For this reason, the transport pipe 14 is required to be thick and short as much as possible. According to Japanese Patent Laid-Open No. 5-271951, in order to reduce the deactivation of radicals, it is necessary to further reduce the surface roughness of the inner surface of the radical transport portion.
However, making the transport pipe 14 for radicals have a smooth inner surface will raise another problem. Using, for example, tungsten for the filament 12 to generate radicals makes the filament 12 serve as a light source at the same time when its temperature rises. The filament 12 generates light having wavelengths from visible light wavelengths to infrared wavelengths, and emits the light in all directions. The light partly enters the inner surface of the transport pipe 14 as shown in FIG. 4. Since the transport pipe 14 is a cylindrical or square pipe, light enters the inner surface at large incident angles. In general, as the incident angle increases, the reflectance at the surface tends to increase. In addition, if the number of times of reflection on the inner surface of the transport pipe remains small, the light leaks out from the transport pipe 14.
The light exiting from the transport pipe 14 at high reflectance and with a small number of times of reflection is applied on optical elements directly or on the housing (for example, the lens barrel) holding the optical elements. When an object is irradiated with light including infrared light (infrared rays) generated by the filament 12, the temperature of the irradiated portion rises to cause thermal expansion. If this object is an optical element, its shape partly changes even though the change is small. If the shape of the object restores until the resumption of exposure, the changed shape influences the orbit of exposure light, resulting in a deterioration in exposure performance. As indicated by reference numerals 17 and 18 in FIG. 4, when unnecessary light is applied in the exposure apparatus, its shape partly distorts to influence the positional relationship between optical elements. When the mirror 7 or the housing 10 is irradiated with infrared light from the filament 12 and the reflecting surface of the mirror 7 changes, the apparatus performs exposure through an optical path 19 shifted from an optical path 16 through which the apparatus should perform exposure, resulting in a deterioration in exposure performance.
Similar problems arise in a charged particle beam (electron beam) drawing apparatus even when a radical generator and a radical transport pipe are installed to remove or reduce contamination on elements constituting a charged particle (electronic) optical system. The infrared light generated from the filament of the radical generator is reflected by the inner surface of the transport pipe and reaches an element of an optical system or the housing holding the element. The portion which the light has reached absorbs the light and then slightly thermally expands. As a result, the shape or position of the element shifts from the design value, and hence the orbit of a charged particle beam for drawing can slightly change. This leads to a deterioration in drawing performance.